The invention relates to an optical coherent receiver having an input for coupling an optical transmission fibre thereto for supplying an optical signal beam, a local oscillator for generating a local oscillator beam, at least one beam-combining element for combining radiation from the signal beam with radiation from the local oscillator and at least one radiation-sensitive detector for converting combined radiation into an electric signal.
Optical coherent receivers using optical heterodyne or homodyne detection are used for optical signal transmission. By mixing the signal beam in a heterodyne or homodyne detection system with an optical beam from a local oscillator, a considerably better result is obtained with regard to the signal-to-noise ratio and the discrimination of background radiation as compared with direct detection of the signal beam.
The principle of coherent detection of optical radiation is extensively described in the article "Optical Heterodyne Detection" by O. E. De Lange in the journal "IEEE Spectrum" of October 1968, pp. 77-85. As has been stated in this article, it is important that the states of polarization of the signal beam and the local oscillator beam correspond as much as possible. A possible solution to achieve this is to split the signal beam into two sub-beams having a mutually perpendicular and fixed direction of polarization. The two sub-beams are then combined with local oscillator radiation which is polarized in the same direction. An alternative is the control of the state of polarization of the signal beam or of the local oscillator beam so that the two states of polarization correspond.
An optical receiver as described in the opening paragraph is known, inter alia from GB-A 2,110,895 which corresponds to U.S. Pat. No. 4,506,388 in which a diversity receiver is described, and from EP-A 0,261,724 describing a receiver using active polarization control.
It has recently been found that it is sometimes possible and sometimes impossible to achieve the sensitivity which, corrected for known interference sources, is theoretically feasible with an optical receiver. In some cases the noise in the detected signal is noticeably larger than the sum of the so-called shot-noise limit, which is the theoretical minimum, and the noise from known error sources. An analysis of this problem has revealed that its source should not be directly sought in the receiver but that the problem is related to the quality of the optical transmission path for the signal between the radiation source and the receiver. Further research has proved that the error source resides in imperfect couplings in the transmission fibre on which radiation coming from the direction of the receiver is partly reflected. This radiation originates from the local oscillator and is reflected in the optical receiver on various surfaces such as the radiation-sensitive surfaces of the detectors, the entrance and exit faces of the beam-combining element and the entrance and exit faces of the connection fibres constituting the radiation paths between the components. As a result a small part of the local oscillator radiation reaches the transmission fibre via the input of the receiver. Although only a very small fraction of the local oscillator radiation is involved, this quantity, related to the intensity of the signal beam, is not negligible. In fact, in the receiver the power of the local oscillator beam is many times larger than that of the signal beam.